Diamond-cooled solid-state laser

ABSTRACT

A diode-pumped solid state laser, e.g., in a side-pumped or end-pumped configuration, including a lasing medium ( 30, 40, 62 ) comprising at least one surface ( 41 ) through which the laser is pumped, and at least one diamond plate ( 32, 42, 64 ) in thermal contact with the at least one surface ( 41 ). In an embodiment a plurality of segments of said lasing medium ( 62 ) are disposed in proximity to each other, and said at least one diamond plate ( 64 ) is disposed between two adjacent segments, and in thermal contact with said elements.

FIELD OF THE INVENTION

The present invention relates to the field of methods for cooling solidstate lasers, especially diode-pumped lasers in side- or end-pumpedconfiguration.

BACKGROUND OF THE INVENTION

One of the primary problems limiting the performance of diode-pumpedsolid state lasers is the ability to remove heat generated within thelasing medium from the pump energy absorbed. Since only a small part ofthe pump energy is converted into laser energy, usually significantlyless than 20%, the majority is absorbed and converted into heat, and hasto be removed from the lasing medium. If the heat is not removedefficiently, the temperature of the lasing medium rises, therebydegrading the efficiency of the lasing action. Furthermore, if thetemperature is not uniformly distributed within the medium, which canresult, inter alia, from poor cooling design, aberrational thermallensing can result, causing degradation and undetermined change of thebeam mode. In addition, the lasing medium can undergo severe mechanicalstress, causing distortion of the laser beam and even a danger of rodfracture. Efficient cooling of the lasing medium, on the other hand,enables higher lasing power to be extracted, and with a higher qualitybeam.

For these reasons, a great deal of effort has been expended on thedevelopment of efficient methods of cooling the lasing medium in solidstate lasers, and especially, because of their more concentrated pumpinput levels, in diode pumped solid state lasers. Commonly used priorart cooling methods for end-pumped lasers include:

-   (a) The use of flowing water in direct contact with the outer    envelope of the laser rod. Throughout this application, and as    claimed, the term “rod” in connection with the lasing medium is    understood to include “slab”, or any other suitable lasing medium    geometry.-   (b) The application of water-cooled blocks of a high conductivity    metal, such as copper, in intimate thermal contact with the lasing    rod or slab, usually with a very thin intermediate layer of indium,    typically 100 μm thick, to improve thermal contact.-   (c) The application to the pumped face of a thin sapphire plate,    whose periphery is cooled, to conduct the heat away from the face.

It is well known that cooling of a lasing slab in the same direction asthe direction of pumping is generally desirable, since the temperaturegradient generated by the heat dissipation is then in the same directionas the axis of cooling. As a result, for a side-pumped slab, geometricalsymmetry of the temperature equipotentials is maintained with respect tothe cooling/pumping direction. The lasing axis is perpendicular to thisdirection, along the length of the slab, and the repeated traverses ofthe intra-cavity beam following a zigzag geometrical path, ensure thatthe beam wavefront passes equally through all parts of the slab, suchthat on average, the beam undergoes no lensing in the zigzag direction.In the direction of the slab cross-section perpendicular to the zigzagdirection, there is no such averaging, but since the temperatureequipotentials are constant in this direction, there is no directionaleffect on the beam as it passes through each “temperature layer”, andthermal lensing is thus minimized.

There are disadvantages to each of the above methods, as follows:

-   (a) The use of direct water-cooling of the laser rod, though very    efficient, significantly increases the complexity of the head    design. Firstly, there is a need to provide water seals at the    relevant locations, with all the concomitant mechanical complexity.    Secondly, there is need to prevent corrosion because of the water.    The head design complexity is further compounded when the head is    such that it is pumped through the face to be cooled, which, as    explained above, is the preferred configuration. Finally, if the    water flow is in direct contact with the laser rod, turbulence    created by the water flow can cause the rod to vibrate, thereby    causing sympathetic beam vibration or wander.-   (b) The use of a water-cooled copper heat sink (or any other    conductive metal) is thermally efficient, because of the high    conductivity of the copper. The major disadvantage of this method,    though, is that the opacity of the copper does not enable pumping of    the laser in the same direction as the cooling direction, as    preferred. Another disadvantage is the need to coat the copper block    with an inert material such as gold, to avoid contamination or    corrosion resulting from the comparatively “dirty” chemical nature    of the copper.-   (c) There is a disadvantage in the use of a peripherally-cooled    sapphire cooling plate in that sapphire has a limited thermal    conductivity, and this limits the pump load that can be born by the    laser rod. For this reason, a peripherally-cooled sapphire cooling    plate needs to be cooled as close as possible to the point of    contact with the lasing medium. For example, in the above-mentioned    Weber article, the sapphire plate is used to cool only the end face    of the laser rod, the end face being of small dimensions, such that    the heat flow path through the sapphire plate is of minimal length.    The length of the rod then also needs its own water cooling    arrangement.

Sapphire plates have also been described in the prior art in use incooling side pumped lasers. In U.S. Pat. No. 5,974,061, to R. W. Byrenet al., an edge pumped laser is described, in which sapphire plates areused as cladding to the wide side of the laser slab, while the slab isside pumped through its narrow edge. The sapphire cladding area iscooled by direct contact with water cooled copper or aluminum blocks,such that the heat flow is across the thickness of the sapphire regions,and similar to the disadvantages mentioned in paragraph (b) above, thepumping cannot be performed through the sapphire-cooled regions.

In U.S. Pat. No. 5,790,575 to J. M. Zamel et al., another side-pumpedlaser using sapphire plates is described, with pumping directed throughthe sapphire plates. However, in this laser, the sapphire plates act astransparent side walls for a water channel disposed between the lasingslab and the sapphire plates, and the lasing slab is effectively cooledby direct contact with flowing water. This design therefore possessesall the disadvantages mentioned in paragraph (a) above, of mechanicalcomplexity and of the effects of water turbulence.

In U.S. Pat. No. 5,317,585, to E. Gregor, there is described a sidepumped solid state laser, incorporating a thin sapphire heat conductingplate, which conducts heat away from the region of the lasing slab toheat sinks located at the peripheries of the plates. The laser is pumpedalong the same axis as that of the heat transfer from the slab. The heatconduction path in the sapphire plate is short, but because of thelimited thermal conductivity of the sapphire, it would appear unlikelythat the laser can be used at the maximum powers which its lasing slabwould be capable of achieving if it were cooled more efficiently. Thisassumption is supported by some other described features of the laser,from which it is apparent that optimum heat conduction was not used.Thus for instance, the thermal contact of the sapphire plate(s) with thelasing slab is not optimized, being executed through “layers oftransparent elastic material”, which inevitably have a compromised valueof thermal conductivity in comparison with solid, heat conductive,materials. In another embodiment, with a cylindrical rod geometry,thermal contact between the lasing rod and the sapphire is achieved bymeans of a cooling channel which contains slow flowing or even staticwater.

It is therefore expected that the use of sapphire heat conduction plateswill result in significant lateral temperature gradients along thelength of the sapphire plates, and hence, poor overall cooling, and pooruniformity of cooling of the lasing slab.

There therefore exists a serious need for a method of coolingside-pumped and end-pumped diode-pumped solid state lasers, whichovercomes the various disadvantages and drawbacks of the prior artcooling methods.

SUMMARY OF THE INVENTION

The present invention seeks to provide a new diode-pumped solid statelaser, which provides for improved cooling efficiency and coolingsymmetry of the lasing rod compared to prior art lasers. The enhancedcooling improves the thermal distribution inside the lasing rod,enabling the construction of high power solid state lasers with improvedefficiency and improved beam quality compared with prior art lasers. Inaddition, the new diode-pumped solid state laser is of simplerconstruction than prior art lasers of similar rating, and has potentialportability. The cooling method according to the present invention,though primarily beneficial to side-pumped lasers, is applicable also toend-pumped lasers.

Thin diamond plates have been used for over 30 years as substrates inthe semiconductor industry for providing good conductivity fordissipating the high heat levels generated in semiconductor devices.Their high thermal conductivity has made them a most preferred materialfor this use. They have also been used, in the same context, for heatsinking diode laser chips. The diamond plates used as semiconductorsubstrates are generally of the non-transparent type, because their costis lower than that of transparent diamonds. In U.S. Pat. No. 5,020,880,to J. Bluege, there is described the use of optically transparentdiamond plates in the output window of a high energy carbon dioxidelaser. However, to the best of the applicants' knowledge, as opposed totheir widespread use to cool semiconductor devices, diamond plates havenever been used to cool optically-pumped solid state lasers.

There is thus provided, in accordance with a preferred embodiment of thepresent invention, a solid state diode-pumped laser, preferably using aslab for the lasing material, and wherein the cooling is performed bymeans of one or more thin plates of diamond intimately attached to oneor more of the sides of the slab, through which side or sides the laseris pumped. Diamond is very transparent throughout the spectral rangeused for pumping, such that pumping can be performed through itvirtually without any loss of pump power. Furthermore, the plates can bepolished optically flat, such that they can be applied to the faces ofthe lasing slab by means of “wringing contact” with light pressure tohold them in place. Good thermal contact can be assured by using a smalldrop of fluid in the interface, which, with minimal pressure, willspread out to leave an exceedingly thin layer in the interface, whichwill practically not affect the heat flow across it. It is believed thatthe fluid thickness is of the order of a micron in thickness. It isunderstood that throughout this application and as claimed, the term“plate” is meant to refer to a piece of material having one dimensionsignificantly thinner than its other two dimensions.

According to other preferred embodiments of the present invention, adiamond plate can also be used on the end of the lasing rod of anend-pumped laser, where the pump energy is input. Its application inthis position in an end-pumped laser is particularly advantageous, sincethe pumping power density in the lasing medium is highest at the inputend face, and it is at this point that the most effective cooling isthus required. The high transparency ensures that no additional heatload is added to the lasing rod assembly.

In addition to its high transparency at typical used pumpingwavelengths, such as in the region of 808 nm, the diamond is also verytransparent at 1.06 μm, the lasing wavelength of Nd:YAG lasers. Acooling window constructed of a thin diamond plate can thus also bepreferably applied to the output end of the lasing rod or slab, inaddition to any other cooling plates used elsewhere on the lasingmaterial.

The thermal conductivity of the diamond is so high, being about fivetimes better than the conductivity of copper, that it is possible toprovide sufficient heat sinking to cool the plate or plates at alocation remote from the region of heat input, which is the area ofcontact with the side of the slab. Because of this exceptionally highthermal conductivity, the diamond plates effectively transfer thecooling effect of the peripherally applied water to the central areas ofthe plate, with a minimal thermal resistance, but without thedisadvantages mentioned above, arising from the direct presence of watercooling in those central areas.

Furthermore, because the heat transfer is so efficient, very thindiamond plates can be used, thus keeping the cost of the plates to aminimum. Furthermore, it becomes possible, according to a furtherpreferred embodiment of the present invention, to cool the periphery ofthe diamond plate or plates by means of forced air, which is usuallyless efficient than water cooling, and therefore not generally used forthe direct cooling of high power solid state laser heads. This enablesthe solid state laser according to the present invention to be operatedwithout the need for attachment to water lines. This embodiment thusopens new possibilities for the convenient use of high power solid statelasers in portable applications, such as in mobile applications.

Furthermore, the simplicity of the structure of a laser head constructedusing diamond plates enables, according to another preferred embodimentof the present invention, the construction of a multiple segment lasinghead, wherein the lasing segments are stacked end to end, with a thindiamond plate between neighboring segments. The diamond plates providean efficient cooling path from each of the segments, and the head can beside or end pumped through the total thickness of the completesegment/plate/segment/plate . . . stack. By this means, a compactmultiple-segment laser rod can be produced with higher combined outputpower than would be attainable by the use of a single laser rod ofsimilar dimensions.

The use of diamond plates thus combine the desired qualities of goodthermal conductivity, significantly better than that provided by theprior art copper cooling slabs, coupled with the optical transparency ofthe prior art sapphire plates.

There is thus provided in accordance with an embodiment of the presentinvention, a diode-pumped solid state laser, e.g., in a side-pumped orend-pumped configuration, including a lasing medium comprising at leastone surface through which the laser is pumped, and at least one diamondplate in thermal contact with the at least one surface.

The diamond plate or plates may preferably be cooled remotely from anarea of thermal contact with the at least one surface. There may also beat least a second diamond plate in thermal contact with a second surfaceof the lasing medium. Either of the diamond plates may be cooled bymeans of convection or conduction (e.g., a cooling fluid, or a cooledmetallic body, or by means of forced air). In the end-pumped embodiment,the lasing beam may be output through the second diamond plate.

In accordance with an embodiment of the present invention, the locationof the diamond plate is such that the direction in which the laser ispumped and the direction in which the laser is cooled are essentiallyco-linear.

Further in accordance with an embodiment of the present invention, aplurality of segments of the lasing medium are disposed in proximity toeach other, and the at least one diamond plate is disposed between twoadjacent segments, and in thermal contact with the segments.

Still further in accordance with an embodiment of the present invention,the laser is side-pumped through at least one of the segments andessentially parallel to the plane of the at least one diamond plate.

Further in accordance with an embodiment of the present invention, thelaser is end-pumped through the plurality of segments and through the atleast one diamond plate. The thicknesses of the segments through whichthe end pumping is performed increase essentially according to the depththrough which the pumping passes. Examples of a suitable lasing mediumare Nd:YAG and Nd:YVO₄.

In accordance with an embodiment of the present invention, the at leastone diamond plate is anti-reflecting at the wavelength at which thelaser is pumped.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which:

FIG. 1 is a schematic view of a side-pumped solid-state laser,constructed and operative according to a preferred embodiment of thepresent invention, with a thin plate of diamond attached in good thermalcontact to one side of the lasing slab;

FIG. 2 is a schematic view of an end-pumped solid-state laser,constructed and operative according to a preferred embodiment of thepresent invention, with a thin plate of diamond attached in good thermalcontact to an end face of the lasing slab;

FIGS. 3A and 3B are graphs showing the temperature equipotentials formedin a laser of the type shown in FIG. 1, wherein in FIG. 3A, the thincooling plate is made of sapphire, and in FIG. 3B, of diamond;

FIG. 4 is a schematic illustration of another preferred embodiment ofthe present invention, similar to that shown in FIG. 2, but wherein thediamond plate is cooled at its periphery by means of forced air; and

FIG. 5 is a schematic drawing of a composite laser rod, constructed andoperative according to another preferred embodiment of the presentinvention, with thin diamond plates inserted between neighboring lasingsegments in the rod, to remove the heat therefrom.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 1, which is a schematic cross-sectionalview of a side-pumped solid-state laser, constructed and operativeaccording to a preferred embodiment of the present invention. To oneside of the lasing slab 30 is applied, in good thermal contact, a thinplate of diamond 32. An area of the contact face of diamond 32 may beoptically polished to provide good thermal contact without necessitatinguse of an intermediate compound. Alternatively or additionally, athermal conductive material may be used as an intermediate compound,wherein such a material may have a refractive index that approximatelymatches a refractive index of diamond 32. For example, improved thermalcontact can be provided by the use of a microscopically thin layer ofcontact fluid 31, preferably an index matching gel, between the slab andthe diamond plate. The diamond plate is cooled at its extremities 34 bymeans of flowing water 36. Alternatively and preferably, the extremitiesof the diamond plate can be embedded in a copper block, operative as aheat sink, and water flowed through the copper block to keep it cool.

Diamond has exceptionally good thermal conduction properties, beingabout 5 times more conductive than the copper slabs previously used inprior art lasers for cooling the sides of the lasing slab, and about twoorders of magnitude more conductive than sapphire. Table I shows thecomparative thermal conductivities of diamond, sapphire and copper, inorder to demonstrate one of the primary bases for the improvement in theoperation of diode-pumped lasers, as engendered by the presentinvention. Other relevant properties shown in table 1 are the absorptioncoefficient at typical pumping wavelengths, and the index of refraction,as required for Fresnel reflection correction. The values for twopreferred lasing media for use in the present invention, Nd:YAG andNd:YVO₄ are also given.

TABLE I Relative internal Thermal transmissivity Index of conductivityMaterial at 808 nm refraction n (W/m. deg K) Diamond High¹ 2.38-2.421900-2200 (from 400 nm- (at 300° K)¹ 10μ)¹ 1500-1600 (at 425° K)¹Sapphire High⁵ 1.76 20.2-18.7 (at 820 nm)⁵ (at 100° F.)² Copper Opaque379-386 (at 173-273° K)³ Nd:YAG Low 1.82 10.5-14 (at 808 nm)⁶ (at293-373° K)⁴ Nd:YVO₄ Low n_(o) = 1.97, 5.1-5.2 n_(e) = 2.19 (at 300° K)⁴(at 808 nm)⁶ ¹De-Beers Ltd catalog on DIAFILM OP ²Handbook of Tables forApplied Engineering Science, by R. Bolz and G Tuve, Chemical Rubber Co.,1970 ³J. P. Holman, Heat Transfer, 7^(th) edition, McGraw-Hill ⁴CasixInc., catalog ⁵Melles Griot Inc., catalog ⁶CVI Inc., catalog

Unlike copper, however, the diamond plate is transparent, so that it nowbecomes possible to side-pump the lasing slab by means of diodes 38positioned in the conventional way above the slab. The laser, accordingto this embodiment of the present invention, thus has the property thatthe direction of the pump light and the direction of cooling areco-axial, with the concomitant advantages of improved lasing symmetryand reduced tendency for thermal lensing and rod distortion, asmentioned above. The conductivity of the diamond plate is so high that avery thin plate suffices to efficiently transport the heat generated inthe lasing slab. Plates of thickness 0.3 mm are preferably used, and canbe obtained from De Beers Industrial Diamond Division, of Shannon, Co.Clare, Ireland. The surfaces may preferably be treated with a wavelengthselective coating to reflect the lasing wavelength and transmit the pumpwavelength, as is known in the art. This increases the pumpingefficiency. Alternatively and preferably, a simple anti-reflectioncoating at either of these wavelengths may be used.

In the embodiment shown in FIG. 1, the bottom side of the slab ismounted in good thermal contact with a water cooled block of copper 39,but it is to be understood that it could equally be cooled by contactwith a second diamond plate, cooled at its extremities, such that itcould be pumped from the bottom face also.

Reference is now made to FIG. 2, which is a schematic side view of anend-pumped solid-state laser, constructed and operative according toanother preferred embodiment of the present invention. The lasing rod 40is cooled from its end face 41 by means of a diamond plate 42 cooled atits peripheries 43 by flowing water 44. The laser is end-pumped 45through the diamond plate. The lasing rod or slab 40 is cooled by directwater contact along its length 46, and also by conduction of heat fromits end face by means of the diamond plate outwards towards the watercooled peripheries 43, as shown by the arrows in the diamond plate. Thepump light 47 is concentrated onto the end face 41 of the rod, throughthe diamond plate 42. The diamond plate is used to provide additionalend face cooling exactly at the location where the pump power density ishighest. The diamond plate thus acts both as a cooling element for theend plate, and as the transparent input window for the pump light. Asthe pump power gets absorbed in its path down the lasing rod, theabsorption within the rod decreases, and the amount of heat that needsto be extracted is reduced. The lasing output beam 48, exits the lasingrod at the far end of the rod towards the output mirror (not shown).

The lasing medium shown in the embodiment of FIG. 2 is a rod, though itis to be understood that the same end-face cooling can be applied alsoto a slab. Furthermore, it is to be understood that, according toanother preferred embodiment of the present invention, both side facediamond plate cooling, as depicted in FIG. 1, and end-face diamond platecooling, as in FIG. 2, can be employed on a single laser.

Reference is now made to FIGS. 3A and 3B, which are respectively, outputplots from computer simulated calculations, showing the coolingabilities of a sapphire plate 51 and a diamond plate 52, used in a laserof the type depicted in FIG. 1. The figures show the temperatureequipotentials formed across the cross-section of a YVO₄ lasing slab 50and along the cooling plate for the two cases, with the same slabdissipation in both cases. The plates in the two cases are of the samedimensions, and are 0.3 mm in thickness. For the purposes of thesimulation, the bottom surface 53 of the slab is maintained at a fixedlow temperature, as if cooled by contact with a water-cooled copperblock. The surfaces of the cooling plate beyond the position of contactwith the slab, 54, are also maintained at a fixed low temperature, as ifalso cooled by contact with cooling water, or a water-cooled heat sink.Because of the symmetry in the X-direction, only the right hand half ofthe lasing head is shown in the two figures.

As is observed in FIG. 3A, where a 0.3 mm thick sapphire window plate 51is used, because of the limited thermal conduction down the length ofthe sapphire plate, there are strong x-axis temperature gradients alongthe length of the cooling plate where it contacts the lasing slab andalong the x-axis of the slab itself. As a result, the desired symmetryof temperature along the pumping and cooling directions is lost. As aconsequence, there will be significant stress within the slab, andthermal lensing because of the temperature gradients along thex-direction. Furthermore, because of the limited thermal conduction downthe sapphire plate, the temperature throughout the slab is higher, andat the center, rises to a significantly high level, 183° C. for thesimulation example shown, thus significantly reducing lasing efficiency.

In contrast to the above results, reference is now made to FIG. 3B,where a 0.3 mm thick diamond window plate is used. It is observed thatthe cooling ability of the diamond plate is so good, that there is nowno discernable temperature gradient, neither along the length of thediamond plate, nor along the x-direction within the lasing slab. Theconduction of the diamond plate is so high that it appears as though thewhole of the top surface of the slab is in good thermal contact with theheat sink. Under such conditions, the flow of pumping energy into theslab in the z-direction, and the flow of heat out into the diamond plateare co-directional and very uniform. This assumes, of course, that theinput pumping energy flow is uniform across the slab, as desired in theideal case. This elimination of temperature gradients along thex-direction of the slab effectively eliminates thermal lensing withinthe slab, thereby improving the beam quality and stability. Furthermore,the overall cooling ability of the diamond plate is so good that themaximum temperature reached in the center of the lasing slab isapproximately 110° C., thus significantly improving laser efficiency andpermissible power output.

Reference is now made to FIG. 4, which is a schematic illustration ofanother preferred embodiment of the present invention, with twodifferences from the embodiment shown in FIG. 1. Firstly, in theembodiment shown in FIG. 4, the lasing slab 30 is pumped from bothsides, and has diode pumping arrays 38 and diamond plates 32 on bothsides of the slab, thus increasing the potential power output of thelasing medium. Secondly, the diamond plates 32 are cooled at theirperiphery by means of forced air. The ends of the diamond plates arepreferably in good thermal contact with aluminum heat sinks 55, as isknown in the art, and the dissipated heat from the lasing medium isremoved from the fins 56 of the heat sinks by means of forced aircooling. The air can be supplied either by built-in fans, or by anair-line supply, or by any other means. The laser of these embodimenthas the advantage of not being dependent on connection to a watersupply, and thus opens up new avenues for portable applications of thepresent invention.

Reference is now made to FIG. 5, which is a schematic side-view drawingof a multiple segment lasing head 60, constructed and operativeaccording to another preferred embodiment of the present invention. Thehead is constructed of lasing segments 62 stacked end to end, with thindiamond plates 64, cooled at their peripheries, inserted betweenneighboring segments. The diamond plates 64 provide an efficient coolingpath from the entire cross section of each of the segments. Theperipheries of the diamond plates are preferably cooled by contact withblocks of copper 68, cooled by means of a flow of water 70. The head ispreferably end-pumped by means of diode-emitted light 66 concentrated onan end face of the stack, or on the first diamond plate, and transmittedthrough the total length of the complete segment/plate/segment/plate . .. lasing stack. By this means, a compact multiple segment laser rod canbe produced, which, because of the efficient cooling means effectivelydispersed through the center of the rod down its length, is capable of ahigher combined output power than would be attainable by the use of asingle laser rod of similar dimensions.

In the preferred embodiment shown in FIG. 5, the lengths of the lasingsegments are varied, tapering from being shorter at the pump end tolonger at the output end of the lasing rod. This is done in order toprovide diamond plate cooling at closer intervals in the region wherethe heat dissipation is highest, i.e. near the pumping input. It is tobe understood, though, that segments of any preferred length may equallybe used in the invention. The difference in refractive indices of thediamond plates and the lasing medium needs to be taken into account whencalculating the mode of the laser cavity, but the high transparency ofthe diamond plates, and their intimate contact with the lasing segments,ensure that the presence of the cooling plates does not interfere withthe quality of the mode. Selective anti-reflection coatings canpreferably be applied to the diamond plates in order to reduce Fresnelreflections, either of the pump light, or of the lasing light.

Alternatively and preferably, the laser head shown in FIG. 5 may be sidepumped by means of diode arrays 72 located opposite the faces of thesegments, or the if they are cylindrical, opposite the outer surfaces ofthe segments. According to yet another preferred embodiment, the laserhead shown in the embodiment of FIG. 5, may be side-pumped andend-pumped.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and subcombinations of various featuresdescribed hereinabove as well as variations and modifications theretowhich would occur to a person of skill in the art upon reading the abovedescription and which are not in the prior art.

1. Apparatus comprising: a diode-pumped, solid state laser comprising alasing medium comprising at least one surface through which said laseris pumped; and at least one diamond plate in thermal contact with saidat least one surface, wherein a plurality of segments of said lasingmedium are disposed in proximity to each other, and said at least onediamond plate is disposed between two adjacent segments, and in thermalcontact with said segments, wherein said laser is end-pumped throughsaid plurality of segments and through said at least one diamond plate,and wherein the thicknesses of said segments through which said endpumping is performed increase essentially according to the depth throughwhich said pumping passes.
 2. Apparatus according to claim 1 whereinsaid at least one diamond plate is cooled remotely from the area of saidthermal contact with said at least one surface.
 3. Apparatus accordingto claim 2, wherein said at least one diamond plate is cooled by meansof at least one of convection and conduction.
 4. Apparatus according toclaim 1 further comprising at least a second diamond plate in thermalcontact with a second surface of said lasing medium.
 5. Apparatusaccording to claim 1 wherein said laser is end-pumped.
 6. Apparatusaccording to claim 5, further comprising a second diamond plate inthermal contact with a surface of said lasing material distant from thatthrough which said laser is pumped.
 7. Apparatus according to claim 6,wherein the lasing beam is output through said second diamond plate. 8.Apparatus according to claim 1 wherein said laser is side-pumped. 9.Apparatus according to claim 1 wherein an area of said at least onediamond plate that is in thermal contact with said at least one surfaceis optically polished.
 10. Apparatus according to claim 1 furthercomprising a layer of a thermal conductive material between said atleast one surface and said at least one diamond plate.
 11. Apparatusaccording to claim 10, wherein said thermal conductive material has arefractive index that approximately matches a refractive index of saidat least one diamond plate.
 12. Apparatus according to claim 1, whereinthe location of said diamond plate is such that the direction in whichsaid laser is pumped and the direction in which said laser is cooled areessentially co-linear.
 13. Apparatus according to claim 1, wherein saidlaser is side-pumped through at least one of said segments andessentially parallel to the plane of said at least one diamond plate.14. Apparatus according to claim 1, wherein said lasing medium isNd:YAG.
 15. Apparatus according to claim 1, wherein said lasing mediumis Nd:YVO₄.
 16. Apparatus according to claim 1, wherein said at leastone diamond plate is anti-reflecting at the wavelength at which saidlaser is pumped.